1. Field of the Invention
The present invention relates to a zoom lens of a large aperture and a large zoom ratio composed, in the order from the object side, of a focusing lens unit of a positive refractive power, a variator lens unit of a negative refractive power, a compensator lens unit of a negative refractive power and a relay lens unit, and more particularly to such zoom lens for use in combination with a television camera.
2. Description of the Prior Art
The optical system for use in a television camera has generally to be a so-called telecentric optical system in which the exit pupil thereof is positioned at an approximately infinite distance. Also a back focus length of a certain magnitude or more is required in order to enable, for example, the mounting of a rear converter. These conditions impose significant restriction on the condition of power of the relay lens unit, and have been obstacles in reducing the size and improving the performance of the zoom lens of the above-mentioned kind.
Also for the purpose of photographing an object at a very close range shorter than the usual shortest photographing distance, there are already known methods of:
(1) integrally moving the lens units positioned in front of the relay lens unit;
(2) moving the variator lens unit and/or the compensator lens unit; and
(3) moving a front group of the relay lens unit. The method (1) is disclosed for example in the Japanese Patents Laid-open No. Sho51-32635 and Sho50-63919, while the method (2) is disclosed for example in the Japanese Patent Publication No. Sho50-23814 and in the Japanese Patents Laid-open Nos. Sho51-2439 and Sho49-53852. These two methods cannot provide satisfactory imaging performance in the very close range photography because of significant variation in the aberrations caused by the relative movement of the variator unit with respect to the relay lens unit. Also in the above-mentioned methods (1) and (2), the position of the entrance pupil varies in the very close range photography since the diaphragm is usually positioned in the relay lens unit or immediately in front thereof. In the very close range photography at the wide angle side, the entrance pupil moves toward the image plane side so that the position of the principal ray corresponding to the maximum image height at the frontmost lens moves away from the optical axis, thus requiring larger lenses in the front units in order to secure enough light in the peripheral area of the image plane. Also the method (1) becomes inevitably complicated in structure, since a separate mechanism is required for integrally moving the focusing lens unit incorporating a focusing mechanism and another lens unit incorporating a zooming mechanism. Such structural complication can be avoided in the method (2) by the use of a groove for moving the lens units at the very close range photography as the extention of a cam groove for zooming, but the photographing at very close range is limited to the wide-angle end or the telescopic end of the zooming range. The very close range photography over the entire zooming range can only be made with a complicated mechanism as in the method (1), and the entire lens system becomes inevitably large since a moving space for such very close range photography has to be provided although it is unnecessary for ordinary zooming operation.
Also a zoom lens for a color television camera requires an exit pupil positioned sufficiently far from the image plane, and the refractive power of the front group of the relay lens unit becomes far larger than that of the rear group if the refractive power arrangement in the relay lens unit is determined to satisfy the above-mentioned condition. Also the variator lens unit and the compensator lens unit positioned in front of the relay lens unit are divergent systems in most cases, so that the diameter of the front group of the relay lens unit is larger than that of the rear group. In the very close range photography with such zoom lens and with the method (3), satisfactory imaging performance cannot be expected due to a significant variation in the aberrations, since a lens unit of a high refractive power, involving strongly diverging and converging light beams, has to be displaced. Also such method, involving the movement of a large lens unit, is disadvantageous in terms of the mechanical precision. A zoom lens capable of photographing at a very close range according to the method (3) is described in the Japanese Patent Publication No. Sho48-32387, but the example is disclosed only in a limited zoom ratio of 2.5 for a zoom lens for an 8-mm movie camera with an f-number 1.8.